In firefighting, water has certain disadvantages which reduce its efficacy in extinguishing fires. The primary effect of water on fire is cooling, thereby reducing the ability of the fuel to burn, and displacement of oxygen necessary for the combustion of fuel. Unfortunately, water has a relatively high surface tension with attendant poor wetting properties for many surfaces. Water also has a low viscosity and flows well at any temperature between its freezing and boiling points. When water is brought into contact with very hot surfaces it has a tendency to bead and roll off the surface. This phenomenon is caused by the formation of a layer of steam between the hot surface and the water which acts to insulate the water from direct contact with the hot surface, reducing the ability of the water to absorb heat from the surface or displace oxygen.
Additives can be introduced to water used for firefighting to reduce the inherent disadvantages of water as a fire extinguishing agent. Surfactants can be added to water to improve its wetting properties. Gelling agents can be added to water to form a gel for use as a firefighting agent. Such gels, however, lose cohesion and viscosity on contact with hot surfaces. Foaming agents combined with water can be quite effective in smothering fires under special conditions. However, foams cannot be used in large fires since high winds created by major fires dissipate the foam or prevent its accurate application. In addition, in the case of chemical fires, burning chemicals frequently cause the collapse of the foam and consequent loss of most of its fire extinguishing properties.
Water retention is important in the curing of hydraulic cements, i.e., cements that are dependent on a hydration reaction for hardening, and concretes that are bound with hydraulic cements. The most common hydraulic cement for construction purposes is Portland cement, which is a heat-treated mixture primarily of calcium carbonate-rich material (such as limestone, marl or chalk) and material that is rich in Al.sub.2.SiO.sub.2 (such as clay or shale). Portland cement comes in several varieties which are distinguished by such characteristics as the rate at which they acquire strength during curing, the amount of heat of hydration that they generate, and their resistance to sulfate attack. Other types of hydraulic cements include aluminous cement, chalcedony cement (made from amorphous quartz) and Roman cement (made by mixing burnt clay or volcanic ash with lime and sand).
The term "concrete" describes to a mixture of stone, gravel or crushed rock and sand (the mixture termed "aggregate") which is bound by a cement. As used herein, the term "concrete" will include reinforced concrete (concrete that contains organic or silica-based fibers or metallic wire, cable or rods as a reinforcing substance) and polymer-cement concrete that is bound with Portland cement and a polymerized monomer or resin system. Hydraulic concrete and cement will be referred to henceforth as "cement". Additional information on the composition and characteristics of cement can be found in Basic Construction Materials (by C. A. Herubin and T. W. Narotta, third edition, Reston Book, Englewood, N.J.) which is incorporated herein by reference.
While the process of curing cement is water dependent, too much water interferes with the process. But, more importantly, too little water results in reduced strength. When cement is freshly poured, the water content may be higher than that which is optimal for proper curing. Thus, some water loss during the curing process can be useful. Often, however, the water loss during the curing process is too great and the cured cement has reduced strength. For instance, a graph on p. 186 of Basic Construction Materials (supra) compares the compressive strength after 180 days of concrete that was moist cured for 28 days vs. 14 days, 7 days, 3 days and 0 days. The results show decreases in compressive strength of approximately 8%, 21%, 32% and 55%, respectively (decreases interpolated from the graph). Excess drying during curing can also lead to the formation of surface cracks.
Consequently, a need to slow the rate of evaporative water loss from curing cement has long been recognized in the art. A previous method to control excessive drying of curing cement has been to apply water as a spray, mist or steam followed by covering the cement with a moisture barrier such as burlap, cotton mats, wet rugs, moist earth or sand, sawdust or other objects likely to act as a moisture barrier. Another method to control excessive drying during curing has been the application of a liquid membrane-forming composition, usually based on natural or synthetic waxes or resins and a volatile carrier solvent, to form, after volatilization of the carrier solvent, a moisture barrier that slows the rate of moisture loss from cement. Water-proof papers and plastic films have also been used.
The curing of cement and the quality of cement obtained is also dependent on temperature, with the recommended range between 55.degree. and 90.degree. F., and on other environmental factors such as wind and rain. In addition to ambient temperature, factors which influence the temperature of cement include the heat of hydration and heat from absorbed sunlight. Wind increases the drying rate of cement and can increase the rate of heat loss, while rain rehydrates cement and can potentially lower the temperature of cement. Of the prior art methods described above, the ones that best address at least one of these environmental factors are the liquid membrane-forming composition, the water-proof paper and the plastic film. These moisture barriers do not afford protection from solar energy or from cold weather.